This invention relates to a nickel-metal hydride storage cell, and more particularly to a composition of a positive electrode active material and a composition of a negative electrode active material.
Nickel-metal hydride storage cells employ a hydrogen-absorbing alloy that is capable of reversibly absorbing and desorbing hydrogen for a negative electrode material. Such nickel-metal hydride storage cells have a larger energy density per unit volume than conventionally used storage cells such as a lead cell and a nickel-cadmium cell, and also exhibit a high degree of tolerance to overcharging and overdischarging as well as an excellent cycle characteristic by its nature. For these reasons, nickel-hydrogen cells have become widely utilized in an electric power supply for a variety of appliances such as portable appliances and electric motors.
Nickel hydroxide is used as a positive electrode active material in such nickel-metal hydride storage cells. In a charge reaction, nickel hydroxide is converted to nickel oxyhydroxide, and in a discharge reaction, nickel oxyhydroxide is converted back to nickel hydroxide. However, when charging is performed at high temperature, oxygen is generated as a side reaction at the same time of the charge reaction by nickel hydroxide, and the charge reaction, in which nickel hydroxide is converted to nickel oxyhydroxide, is thereby impeded. As a result, a utilization factor of nickel hydroxide is reduced, causing a degradation in a positive electrode capacity.
In view of this problem, as a method for suppressing such a side reaction, for example, Japanese Unexamined Patent Publication Nos. 5-28992, and 6-103973 disclose a technique in which an yttrium compound or the like is added to a nickel positive electrode. According to the technique disclosed therein, an yttrium compound or the like is adsorbed on the surface of nickel oxide, and serves to increase an overvoltage, which is a competitive reaction in charging at high temperature, leading to a sufficient charge reaction of nickel hydroxide to nickel oxyhydroxide. Hence, a utilization factor of the positive electrode active material at high temperature is improved.
However, in a nickel-metal hydride storage cell, when charged at high temperature, a side reaction of a dissociation of hydrogen takes place in the negative electrode as well as the positive electrode. This side reaction causes a reduction in a utilization factor of the negative electrode active material, resulting in a decrease in a negative electrode capacity. The dissociated hydrogen also causes a decrease in a positive electrode capacity. Accordingly, even if a utilization factor of the positive electrode active material is improved by employing the above-described technique, a cell capacity as a whole cannot be sufficiently improved, since the cell capacity is limited by the negative electrode in which the capacity is degraded, or the dissociated hydrogen is absorbed in the positive electrode, causing a decrease in a positive electrode capacity.
The present invention intended to solve the foregoing problems. It is an object of the present invention to provide a nickel-metal hydride storage cell in which both a utilization factor of a positive electrode active material and a utilization factor of a negative electrode active material are improved at high temperature, a high capacity is maintained at high temperature, and an excellent cycle characteristic is achieved. In order to accomplish this and other objects of the invention, the following configurations are provided in a group of the invention.
(1) First Group of the Invention.
The present invention provides a nickel-metal hydride storage cell comprising in a cell-case, a positive electrode comprising a positive electrode active material composed mainly of nickel hydroxide powder, a negative electrode comprising a negative electrode active material composed mainly of hydrogen-absorbing alloy powder, and a separator interposed between the positive and negative electrodes and impregnated with an electrolyte, the nickel-metal hydride storage cell characterized in that the negative electrode active material contains a copper compound, and the positive electrode active material contains at least one compound selected from the group consisting of bismuth compound, calcium compound, ytterbium compound, manganese compound, copper compound, scandium compound, and zirconium compound.
In accordance with the configuration set forth above, the compound contained in, the positive electrode, such as bismuth and the like, serves to accomplish a sufficient charge reaction of nickel hydroxide to nickel oxyhydroxide at high temperature. The copper compound contained in the negative electrode serves to suppress a decrease in a utilization factor of the negative electrode active material (hydrogen-absorbing alloy) at high temperature, and to prevent a degradation of the negative electrode capacity. Therefore, it is made possible to prevent such undesirable effects that a cell which is initially positive-electrode-limited changes into a negative-electrode-limited cell at high temperature, and that a cell capacity is limited by the negative electrode having a smaller capacity. In other words, with the above-described configuration, an increase of the positive electrode capacity entirely leads to an increase of a cell capacity (a capacity which can be outputted from a cell as a whole). Therefore, it is made possible to solve such a problem in prior art that a sufficient cell capacity cannot be achieved even if the positive electrode capacity is improved.
In the above-described configuration, the at least one compound may be an oxide or hydroxide of one of bismuth, calcium, ytterbium, manganese, copper, scandium, and zirconium.
In addition, the copper compound may be an oxide or hydroxide of copper.
In addition, the positive electrode active material may comprise an aggregate of coated particles each in which a coating layer comprising a sodium-containing cobalt compound is formed on a surface of a nickel hydroxide particle, and the positive electrode active material may be such that the oxide or hydroxide of one of bismuth, calcium, ytterbium, manganese, copper, scandium, and zirconium, is added to the aggregate of coated particles.
In the configuration of employing the coated particles each particle in which a coating layer comprising a sodium-containing cobalt compound is formed on a surface of on a nickel hydroxide particle, the cobalt compound exists on the surface of the nickel hydroxide particle, and thereby electrical conductivity in the active material is effectively improved. As a result, the addition of a cobalt compound is minimized, and a proportion of nickel hydroxide (active material) in the positive electrode active material is made sufficient. In other words, in this configuration, it is possible to suppress a degradation in a theoretical capacity caused by the addition of cobalt compound, and thus an increase of electrical conductivity (an increase in an active material utilization factor) leads to a significant increase in an actual capacity of the positive electrode.
In addition, in the positive electrode active material in which an oxide or a hydroxide of such an element as bismuth and the like is added to the aggregate of the coated particles, a compound such as an yttrium and the like serves to suppress a diffusion of the cobalt compound into the inside of the nickel hydroxide particle, thus preventing such an occurrence that a concentration of the cobalt compound on the surface of the nickel hydroxide decreases as charge-discharge cycles proceed. Hence, by these effects, it is made possible to significantly improve a cell capacity, which is a total of the positive electrode performance and the negative electrode performance, as well as a cycle characteristic of the cell.
(2) Second Group of the Invention
The present invention also provides a nickel-metal hydride storage cell comprising in a cell case, a positive electrode comprising a positive electrode active material composed mainly of nickel hydroxide powder, a negative electrode comprising a negative electrode active material composed mainly of hydrogen-absorbing alloy powder, and a separator interposed between the positive and negative electrodes and impregnated with an electrolyte, the nickel-metal hydride storage cell characterized in that the negative electrode active material contains a copper compound, and the positive electrode active material is such that an yttrium compound is added to an aggregate of coated particles each in which a coating layer comprising a sodium-containing cobalt compound is formed on a surface of a nickel hydroxide particle.
The above-described configuration makes it possible to enhance an effect of suppressing the diffusion of cobalt compound into the inside of a nickel hydroxide particle and an effect of suppressing the side reaction on the surface of the nickel hydroxide particle, both effects caused by the yttrium compound. Therefore, a cell capacity at high temperatures and a cycle characteristic of the cell are more significantly improved.
In the above-described configuration of the cell, the yttrium compound may be an oxide or hydroxide of yttrium.
In addition, an amount of the yttrium compound to be contained may be 0.2 to 10 wt. % based of the weight of the positive electrode active material.
In addition, the copper compound may be an oxide or hydroxide of copper.
In addition, an amount of the copper compound may preferably be 0.5 to 20 wt. % based on the weight of the negative electrode active material.
Now, a more specific explanation for the above-described configurations is given below.
It is preferable that a copper compound to be added to the negative electrode active material as its constituting element be an oxide of copper such as Cu2O, or a hydroxide of copper such as Cu(OH)2. When such an oxide or hydroxide of copper is employed, in an electric potential range in which the hydrogen-absorbing alloy electrochemically absorbs and desorbs hydrogen in an alkaline electrolyte, the copper compound is made to exist in the form of metallic copper, which is highly conductive. Therefore, the. copper compound effectively functions as a conductivity enhancer. Thus, a utilization factor of the negative electrode active material is greatly increased by employing the oxide or the hydroxide of copper.
The amount of the copper compound to be added is preferable to be 0.5 to 20 wt. % based on the weight of the active material. If the amount of the copper compound is less than 0.5 wt. % based on the weight of the active material, a sufficient increase of conductivity cannot be attained. By contrast, if the amount of the copper compound is more than 20 wt. %, the effect of increasing conductivity by adding a copper compound is hindered by an adverse effect of decreasing the amount of hydrogen-absorbing alloy to be contained. In both cases, the effect of increasing conductivity by adding the copper compound and the effect of increasing a cell capacity at high temperature are not sufficiently achieved.
For the compound of one of bismuth, calcium, ytterbium, manganese, copper, scandium, zirconium, and yttrium to be added as a constituting element of the positive electrode, it is preferable to employ an oxide or a hydroxide of such elements, and more preferable to employ an oxide or a hydroxide of yttrium. The reason is that when an oxide or hydroxide of such elements is employed, even if an ionically dissociated substance other than the metal in the oxide or hydroxide of such elements is bonded with other components in the alkaline electrolyte, the resulting product by such a reaction causes little adverse effect on the cell reaction. Also, the oxide or hydroxide of yttrium is particularly preferable since the oxide or hydroxide of yttrium exhibits excellent effects of increasing an overvoltage and of preventing a diffusion of cobalt from the coating layer on the nickel hydroxide into the inside of nickel hydroxide.
Examples of the above-mentioned oxide of such elements as bismuth and the like include Bi2O3, CuO, Sc2O3, ZrO2, Yb2O3, and MnO2. Examples of the above-mentioned hydroxide include Ca(OH)2, Bi(OH)3, Cu(OH)2. Examples of the oxide or hydroxide of yttrium include Y2O3, and Y(OH)3.
The amount of the yttrium compound to be added as a constituting element of the positive electrode active material is preferable to be 0.2 to 10 wt. % based on the weight of the active material. When the amount of the yttrium compound is 0.2 to 10 wt. %, the yttrium compound effectively functions and consequently a utilization factor of the positive electrode active material increases. By contrast, if the amount of the yttrium compound is less than 0.2 wt. % of the weight of the active material, a diffusion of cobalt compound into the inside of the nickel hydroxide particle cannot be sufficiently suppressed. If the amount of the yttrium compound is more than 10 wt. %, an energy density per a weight of the active material is reduced by an adverse effect of decreasing a proportion of nickel hydroxide in the active material, and thereby the positive electrode capacity cannot be sufficiently increased.
It is to be noted that in a nickel-metal hydride storage cell according to the present invention, it is possible that compounds in the positive and negative electrodes (such as a copper compound in the negative electrode and a bismuth compound, a yttrium compound or the like in the positive electrode) exist as a simple substance of a metal after the cell is assembled. This is because such compounds in a cell change their forms by being oxidized and reduced by charging and discharging.
It is also noted that examples of hydrogen-absorbing alloy used in the present invention include a rare-earth element type hydrogen-absorbing alloy, a Zrxe2x80x94Ni type hydrogen-absorbing alloy, a Tixe2x80x94Fe type hydrogen-absorbing alloy, a Zrxe2x80x94Mn type hydrogen-absorbing alloy, a Tixe2x80x94Mn type hydrogen-absorbing alloy, and a Mgxe2x80x94Ni type hydrogen-absorbing alloy. In addition, other elements such as types of materials for a core for the positive and negative electrodes (a current collector) and a separator, compositions for the alkaline electrolyte, and the like are not particularly limited, and various types of known materials used for-nickel-metal hydride storage cells may be used. In addition, various types of cell structures may be employed for the cell according to the present invention.